Cultured skin fibroblasts from patients with I-cell disease, an inherited lysosomal disorder characterized by multiple acid hydrolase deficiency, were assayed with respect to their intracellular content of the amino acid cystine. Seven of these lines were found to contain abnormally elevated levels of this amino acid in a manner reminiscent of the unrelated lysosomal storage disorder cystinosis. Several lines from patients with mucolipidosis III, a biochemically similar but clinically less severe variant of I-cell disease, were normal with respect to their intracellular cystine levels. The finding of supra-normal levels of cystine in I-cell fibroblasts may be relevant to an understanding of the pathogenesis of human cystinosis.